422 research outputs found
Enhanced Production of Neutron-Rich Rare Isotopes in Peripheral Collisions at Fermi Energies
A large enhancement in the production of neutron-rich projectile residues is
observed in the reactions of a 25 MeV/nucleon 86Kr beam with the neutron rich
124Sn and 64Ni targets relative to the predictions of the EPAX parametrization
of high-energy fragmentation, as well as relative to the reaction with the less
neutron-rich 112Sn target. The data demonstrate the significant effect of the
target neutron-to-proton ratio (N/Z) in peripheral collisions at Fermi
energies. A hybrid model based on a deep-inelastic transfer code (DIT) followed
by a statistical de-excitation code appears to account for part of the observed
large cross sections. The DIT simulation indicates that the production of the
neutron-rich nuclides in these reactions is associated with peripheral nucleon
exchange. In such peripheral encounters, the neutron skins of the neutron-rich
124Sn and 64Ni target nuclei may play an important role. From a practical
viewpoint, such reactions between massive neutron-rich nuclei offer a novel and
attractive synthetic avenue to access extremely neutron-rich rare isotopes
towards the neutron-drip line.Comment: 4 pages, 3 figures, submitted to Phys. Rev. Let
SIGMA and XTE observations of the soft X-ray transient XTEJ1755-324
We present observations of the X-ray transient XTEJ1755-324 performed during
summer 1997 with the XTE satellite and with the SIGMA hard X-ray telescope
onboard the GRANAT observatory. The source was first detected in soft X-rays
with XTE on July 25 1997 with a rather soft X-ray spectrum and its outburst was
monitored in soft X-rays up to November 1997. On September 16 it was first
detected in hard X-rays by the French soft gamma ray telescope SIGMA during a
Galactic Center observation. The flux was stronger on September 16 and 17
reaching a level of about 110 mCrab in the 40-80 keV energy band. On the same
days the photon index of the spectrum was determined to be alpha =-2.3 +/- 0.9
(1 sigma error) while the 40-150 keV luminosity was about 8 x 10^{36} erg/s for
a distance of 8.5 kpc. SIGMA and XTE results on this source indicate that this
source had an ultrasoft-like state during its main outburst and a harder
secondary outburst in September. These characteristics make the source similar
to X-Nova Muscae 1991, a well known black hole candidate.Comment: 19 pages LaTeX, 6 Postscript figures included, Accepted by
Astrophysical Journa
GOHTAM: a website for âGenomic Origin of Horizontal Transfers, Alignment and Metagenomicsâ
Motivation: This website allows the detection of horizontal transfers based on a combination of parametric methods and proposes an origin by researching neighbors in a bank of genomic signatures. This bank is also used to research an origin to DNA fragments from metagenomics studies
Properties Of The Hard X-ray Radiation From The Black Hole Candidates: Cygnus X-1 And 1E1740.7-2942
The entire dataset of the GRANAT/SIGMA observations of Cyg X-1 and
1E1740.7-2942 in 1990-1994 was analyzed in order to search for correlations
between primary observational characteristics of the hard X-ray (40-200 keV)
emission - hard X-ray luminosity, hardness of the spectrum (quantified in terms
of the best-fit thermal bremsstrahlung temperature kT) and the RMS of
short-term flux variations. Although no strict point-to-point correlations were
detected certain general tendencies are evident. It was found that for Cyg X-1
the spectral hardness is in general positively correlated with relative
amplitude of short-term variability. The correlation of similar kind was found
for X-ray transient GRO J0422+32 (X-ray Nova Persei 1992). For both sources an
approximate correlation between kT and L_X was found. At low hard X-ray
luminosity - below 10E37 erg/sec - kT increases with L_X. At higher luminosity
the spectral hardness depends weaker or does not depend at all on the hard
X-ray luminosity. The low luminosity end of these approximate correlations (low
kT and low RMS) corresponds to extended episodes of very low hard X-ray flux
occurred during SIGMA observations.Comment: 6 pages, 7 figures, uses mn.sty, epsf.sty, psfig.tex; Accepted for
publication in MNRAS; Also available at
http://hea.iki.rssi.ru/~sik/PAPERS/mnras97.ps.gz (gzipped PostScript
A Cherenkov imager for charge measurements of Nuclear Cosmic Rays in the CREAM II instrument
A proximity focusing Cherenkov imager for the charge measurement of nuclear cosmic rays in the CREAM II instrument, called CHERCAM, is under construction. This imager consists of a silica aerogel radiator plane facing a detector plane equipped with standard photomultipliers. The two planes are separated by a minimal ring expansion gap. The Cherenkov light yield is proportional to the squared charge of the detected particle. The expected relative light collection accuracy is in the few percents range. It should lead to single element separation over the range of nuclear charge Z of main interest 1 Z \approx$ 26
First detection of a VHE gamma-ray spectral maximum from a Cosmic source: H.E.S.S. discovery of the Vela X nebula
The Vela supernova remnant (SNR) is a complex region containing a number of
sources of non-thermal radiation. The inner section of this SNR, within 2
degrees of the pulsar PSR B0833-45, has been observed by the H.E.S.S. gamma-ray
atmospheric Cherenkov detector in 2004 and 2005. A strong signal is seen from
an extended region to the south of the pulsar, within an integration region of
radius 0.8 deg. around the position (RA = 08h 35m 00s, dec = -45 deg. 36'
J2000.0). The excess coincides with a region of hard X-ray emission seen by the
ROSAT and ASCA satellites. The observed energy spectrum of the source between
550 GeV and 65 TeV is well fit by a power law function with photon index = 1.45
+/- 0.09(stat) +/- 0.2(sys) and an exponential cutoff at an energy of 13.8 +/-
2.3(stat) +/- 4.1(sys) TeV. The integral flux above 1 TeV is (1.28 +/- 0.17
(stat) +/- 0.38(sys)) x 10^{-11} cm^{-2} s^{-1}. This result is the first clear
measurement of a peak in the spectral energy distribution from a VHE gamma-ray
source, likely related to inverse Compton emission. A fit of an Inverse Compton
model to the H.E.S.S. spectral energy distribution gives a total energy in
non-thermal electrons of ~2 x 10^{45} erg between 5 TeV and 100 TeV, assuming a
distance of 290 parsec to the pulsar. The best fit electron power law index is
2.0, with a spectral break at 67 TeV.Comment: 5 pages, 4 figures, accepted for publication in Astronomy and
Astrophysics letter
PaleofitogeografĂa de los pinares en las montañas perifĂ©ricas de la cuenca del Duero
En las dos Ășltimas dĂ©cadas se han obtenido numerosos resultados procedentes de diferentes trabajos paleobotĂĄnicos (Cuaternario final) realizados en los territorios montanos perifĂ©ricos de la depresiĂłn del Duero. Las metodologĂas empleadas han sido diversas y tienen que ver con los diferentes tipos de yacimientos y de muestras biolĂłgicas seleccionadas para su estudio; entre ellas destacan las tĂ©cnicas de microscopĂa Ăłptica para la identificaciĂłn de maderas subfĂłsiles, la morfologĂa comparada de macrorrestos y los estudios dendrocronolĂłgicos en ĂĄrboles longevos y maderas subfĂłsiles. En esta comunicaciĂłn se reĂșnen y resumen los trabajos mĂĄs importantes, haciĂ©ndose una sĂntesis de las conclusiones obtenidas en los trabajos paleofitogeogrĂĄficos realizados hasta el momento; se hace hincapiĂ© en la informaciĂłn proporcionada por los macrorrestos (maderas, estrĂłbilos) asĂ como por la recogida en los registros dendrocronolĂłgicos (con extensiĂłn a la dendroecologĂa). Otro de los objetivos de la comunicaciĂłn es la sĂntesis de las conclusiones obtenidas en los trabajos paleofitogeogrĂĄficos realizados hasta el momento. Uno de esos resultados es que el comportamiento de los pinares a lo largo del Holoceno presenta diferentes modalidades en cada una de las cordilleras que bordean la cuenca del Duero; desde casos con marcada estabilidad a otros en que se muestran variaciones temporales apreciables (con patrones de heterogeneidad en funciĂłn de un eje N-S y/o O-E
A possible association of the new VHE gamma-ray source HESS J1825--137 with the pulsar wind nebula G18.0--0.7
We report on a possible association of the recently discovered very
high-energy -ray source HESS J1825--137 with the pulsar wind nebula
(commonly referred to as G 18.0--0.7) of the year old
Vela-like pulsar PSR B1823--13. HESS J1825--137 was detected with a
significance of 8.1 in the Galactic Plane survey conducted with the
H.E.S.S. instrument in 2004. The centroid position of HESS J1825--137 is offset
by 11\arcmin south of the pulsar position. \emph{XMM-Newton} observations have
revealed X-ray synchrotron emission of an asymmetric pulsar wind nebula
extending to the south of the pulsar. We argue that the observed morphology and
TeV spectral index suggest that HESS J1825--137 and G 18.0--0.7 may be
associated: the lifetime of TeV emitting electrons is expected to be longer
compared to the {\it XMM-Newton} X-ray emitting electrons, resulting in
electrons from earlier epochs (when the spin-down power was larger)
contributing to the present TeV flux. These electrons are expected to be
synchrotron cooled, which explains the observed photon index of , and
the longer lifetime of TeV emitting electrons naturally explains why the TeV
nebula is larger than the X-ray size. Finally, supernova remnant expansion into
an inhomogeneous medium is expected to create reverse shocks interacting at
different times with the pulsar wind nebula, resulting in the offset X-ray and
TeV -ray morphology.Comment: 5 pages, 3 figures, to appear in Astronomy and Astrophysics Letter
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